The Present invention relates to a method for manufacturing a filter utilizing a porous ceramic membrane (referred to as a porous membrane hereinafter) as a separation film. The present invention particularly relates to a method for manufacturing a filter having a uniform film thickness and smooth film face, as well as a porous membrane having a sharp micro-pore size distribution.
Filters utilizing a ceramic porous membrane as separation films (i.e., filtration membranes) are more useful as solid-liquid separation filters when compared with filters utilizing a polymer membrane as a separation film. In addition to the advantage that the micro-pore size of the porous ceramic membrane which determines filtration ability is precisely controllable, the ceramic filter is highly reliable due to its excellent physical strength, durability, and high corrosion resistance to acids or alkalis.
The filter that has been frequently used comprises a ceramic porous membrane formed on a porous substrate. Typically, the ceramic porous membrane has a far finer pore size than the porous substrate to improve filtration performance while maintaining a given water permeation rate.
The filter described above may be manufactured according to conventional methods by depositing a slurry containing framework particles to form a film, such as a dipping method, followed by firing the deposited film. However, the inventors of the present invention have disclosed an excellent slurry deposition methodxe2x80x94a filtration deposition methodxe2x80x94by which film defects such as pin holes can be prevented from being generated, in Japanese Examined Patent Application Publication No. 63-66566.
In the above filtration deposition method, the face of the porous substrate to be provided with a separation film is isolated to be airtight from the face of the porous substrate not provided with the separation film in a vacuum chamber, after substituting air in the fine pores inside of the porous substrate with a liquid. Then, a film deposition slurry containing ceramic framework particles is allowed to contact the face of the porous substrate to be provided with the separation film by continuously feeding the slurry thereon, and a differential filtration pressure is applied between the face of the porous substrate to be provided with the separation film and the other face, thereby depositing a slurry film on the desired surface of the porous ceramic substrate.
In one example, a slurry is deposited on an inner wall of a through-hole of a porous substrate prepared by forming a single through-hole along the longitudinal direction of a cylinder (referred to as a tubular substrate hereinafter). As shown in FIG. 4, the inside and the outer circumference side of the through-hole of the porous substrate 1, whose fine pores are filled with a liquid, are secured with flanges 2 and 3, and bolts 5 in an apparatus comprising a vacuum chamber 6, a reservoir 8, a pump 7, the flanges 2 and 3, and a tubing 10, so that one side is isolated from the other side to be airtight.
Subsequently, the inside of the vacuum chamber 6 is evacuated with a vacuum pump 13 so that the pressure at the outer circumference side of the porous substrate 1 is reduced, while allowing a slurry 9 in the reservoir 8 to contact the inner wall 12 of the through-hole by continuously feeding the slurry into the through-hole of the porous substrate 1. A slurry film is deposited on the inner wall 12 of the through-hole of the porous substrate 1 by the operation as described above, because a differential filtration pressure is applied between the outer circumference side of the porous substrate 1 and the inner wall 12 of the through-hole, and water in the slurry is discharged as a filtrate from the outer circumference side of the porous substrate 1.
Film defects such as pin-holes may be prevented in the filtration deposition method as described above, since a pre-treatment for substituting the remaining air in the fine-pores of the porous substrate 1 is applied. Continuously feeding the slurry into the through-hole permits a deposition film having a uniform thickness and quality to be formed because framework particles in the slurry are slowly deposited. Accordingly, a better quality filter may be obtained as compared with a filter obtained by a film deposition method such as dipping.
However, the advantage of the filtration deposition method that a film with a uniform thickness and quality can be formed is apt to be offset due to the recent tendency to make the filtration area, or the filter size itself, large for the purpose of improving the filtration ability of the filter.
A first problem arises when the porous substrate is a long size tube, with a length of 50 cm or more, comprising a through-hole formed along the longitudinal direction of the cylinder.
When the length of the porous substrate is large, the film deposition proceeds more easily at the slurry feed side than at the slurry discharge side, where the film deposition hardly proceeds. As shown in FIG. 5, it was difficult to deposit the slurry in the through-hole with a uniform film thickness.
A second problem arises when a filter is manufactured comprising a plurality of through-holes provided in parallel to one another along the longitudinal direction of the cylinder (referred to as a revolver magazine type substrate hereinafter) and separation films are formed on the inner walls of the respective through-holes, in order to increase the filtration area per unit volume and enhance overall filtration performance.
When the filtration deposition method is applied to the porous revolver magazine type substrate, film deposition is easily facilitated in the through-holes located at the outer circumference side of the plural through-holes as shown in FIG. 6, whereas the films are hardly deposited in the through-holes located at or near the center of the porous substrate. Consequently, it is difficult to form slurry deposition films with a uniform film thickness in all the through-holes. Further, the slurry deposition film is formed having larger thickness at the outer circumference side of the porous substrate even in one through-hole.
Although the second problem may be partly solved by increasing the thickness of the slurry deposition film, the film thickness in all the through-holes cannot be made uniform by any means. In addition, this method is not preferable since the porous membrane formed after firing has a large film thickness, resulting in a lowering water permeation rate (or filtration ability) of the filter.
A third problem is that the surface of the porous membrane is roughened as shown in FIG. 7 irrespective of the configuration of the substrate, or the micro-pore size distribution of the porous membrane turns out to be broad.
Accordingly, it is an object of the present invention to solve the foregoing problems by providing a method for manufacturing a filter having a uniform film thickness, smooth film surface and sharp micro-pore size distribution.
The technical problems of the conventional art as described above arise from the fact that the differential pressure applied on the substrate during deposition of the filtration film varies depending on the sites on the substrate. The inventors of the present invention have found that this problem may be solved by adding an organic polymer to the film deposition slurry to endow the deposition film layer with filtration resistance.
In one aspect, the present invention provides a method for manufacturing a filter utilizing a porous ceramic membrane as a separation film comprising the steps of substituting the air inside the fine-pores of the porous substrate with a liquid, isolating the face of the porous substrate face to be provided with a separation film from the other face of the porous substrate on which a separation film is not provided so as to enable a pressure differential therebetween, continuously feeding a film deposition slurry containing ceramic framework particles into contact with the face of the porous substrate to be provided with the separation film, applying a differential filtration pressure between the face of the porous substrate to be provided with the separation film and the other face of the porous substrate, and depositing the slurry on the desired face of the porous substrate, wherein an organic polymer for endowing the deposition film layer with filtration resistance is added to the slurry.
Preferably, the method uses a cylindrical porous substrate containing a single or a plurality of through-holes formed along the longitudinal direction of the cylinder, and comprises the steps of isolating the inner side of the through-holes of the porous substrate from the outer circumference side of the porous substrate to enable a pressure differential therebetween, continuously feeding the film deposition slurry into the through-holes to allow the slurry to contact the inner walls of the through-holes, applying a differential filtration pressure between the inner side of the through-holes of the porous substrate and the outer circumference side of the porous substrate, and depositing the slurry on the inner walls of the through-holes of the porous substrate.